Pulse-width modulation (PWM) is commonly used to control a switching regulator in which a modulator generates a pulse-width-modulated control signal (SPWM) at a fixed frequency (fPWM)). An advantage of using PWM is to enable a switching regulator to provide an output voltage (VOUT) in which all its spectral components are located at multiples of fPWM). As illustrated in FIG. 1, spectral components of VOUT are all located at multiples of fPWM, such as from fPWM) to αfPWM, where α is a positive integer, regardless of value of the load current (ILOAD) delivered by a switching regulator. Predictable spectrum at different levels of ILOAD is thus achieved by PWM, which is very important to switching regulators used to power spectrum-sensitive circuits, such as communication circuits. However, the use of MHz-range fPWM in conventional PWM modulators to enable the use of small inductive and capacitive elements in the switching regulator inevitably degrades its light-load efficiency. This is because switching loss of a switching regulator is proportional to fPWM, but not to ILOAD delivered by the switching regulator. As ILOAD is reduced, switching loss dominates and becomes much larger than power used by the load connected to the switching regulator. As shown in FIG. 2A, power efficiency of a switching regulator with PWM at MHz-range fPWM is significantly decreased as ILOAD is reduced.
Pulse-frequency modulation (PFM) is known as an effective method to improve light-load efficiency of a switching regulator. A PFM modulator generates a control signal (SPFM) in which frequency of SPFM (fPFM) is monotonically reduced with decreased ILOAD. One of the possible relationships between fPFM and ILOAD is shown in FIG. 2B. Switching loss of a switching regulator with PFM can be reduced with decreased ILOAD such that, as shown in FIG. 2A, light-load efficiency of a switching regulator with PFM is much higher than that with PWM. However, PFM makes a switching regulator with a VOUT in which all its spectral components depend on the value of ILOAD. Spectrum of switching regulator with PFM becomes unpredictable at different levels of ILOAD such that a switching regulator with PFM is not favorable to power spectrum-sensitive circuits, such as communication circuits.
As a result, there is a need for a modulator which is able to simultaneously facilitate a switching regulator to have smaller-size inductive and capacitive elements, improved power efficiency at light load, and predictable spectrum at different levels of load.